1. Field of Invention
This invention provides a bifacial photovoltaic device structure to absorb lights coming from either side.
2. Prior Arts
Since the first published photovoltaic effect by a French scientist Edmund Becquerel in 1839, the quest to improve photovoltaic cell performance has begun.
Through better light absorption, U.S. Pat. No. 2,001,672 to Carpenter (1930) discloses ways to increase solar cell efficiency by incorporating a back reflector layer. Since then, much efforts have been focusing on improving single-sided solar cell with one light absorption face (for example, U.S. Pat. No. 3,990,101 to Ettenberg et al. (1976), U.S. Pat. No. 4,196,623 to Shay et al. (1980), U.S. Pat. No. 4,355,196 to Chai (1982), and U.S. Pat. No. 7,053,294 to Tuttle et al. (2006)).
In circumstance when light shines on the back side of such single-sided solar cells, no electricity would be generated. This poses problems to applications employing solar cells that cannot move to track lights. Although it is possible to incorporate a light tracking mechanism to solar cell panels, the adoption of such light tracking mechanism increases cost of the electricity generation system.
U.S. Pat. No. 3,278,811 to Mori (1966) discloses a radiant energy transducer which is sensitive to radiation incident on either side of the transducer. However, given electron's limited diffusion length in semiconductors, thickness of the device has to be very small. Hence, such design is not practical in larger surface area photovoltaic application. Otherwise, device has to use complicated material structure as disclosed in U.S. Pat. No. 3,948,682 to Bordina et al. (1976) which is costly to produce.
Process improvement has been described by R. Hezel “Novel Back Contact Silicon Solar Cells Designed for Very High Efficiencies and Low-cost Mass Production” (2002) to lower cost of production. However, his device structure still requires semiconductor material to be selectively added and subtracted in high precision which are costly. In addition, devices based upon bulk silicon semiconductor material, disclosed in U.S. Pat. No. 6,403,877 to Katsu (2002), are heavy and fragile to handle.
Alternatively, semiconductor layers in a bifacial solar cell can be formed simultaneously as described by D. L. Young, J. Abushama, R. Noufi, X. Li, J. Keane, T. A. Gessert, J. S. Ward, M. Contreras, M. Symko-Davies and T. J. Coutts “A New Thin-Film CuGaSe2/Cu(In,Ga)Se2 Bifacial, Tandem Solar Cell with Both Junctions Formed Simultaneously” (2002). However, their structure requires four electrical terminals thus making it cumbersome and costly to interconnect.
From the above description, it is evident that novel bifacial photovoltaic cells with simplified structure for interconnection and fabrication will be valuable for photovoltaic applications.